Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (MEA). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, normally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
As the fuel gas supplied to the solid oxide fuel cell, normally, a hydrogen gas, CO, or methane generated from hydrocarbon raw material by a reformer is used. In general, in the reformer, a reformed raw material gas is obtained from hydrocarbon raw fuel of a fossil fuel or the like, such as methane or LNG, and the reformed raw material gas undergoes steam reforming, partial oxidation reforming, or autothermal reforming to produce a reformed gas (fuel gas).
In this type of the fuel cell, it is required to improve the performance of tracking the load at the time of load reduction. For example, a method of controlling a fuel cell power generation apparatus as disclosed in Japanese Laid-Open Patent Publication No. 07-022045 is known. As shown in FIG. 9, the fuel cell power generation apparatus includes a fuel cell 3 for performing power generation using an anode gas 1a containing hydrogen and a cathode gas 2a containing oxygen, a reformer 5 for burning an anode exhaust gas 1b discharged from the fuel cell 3 using some of a cathode exhaust gas 2b discharged from the fuel cell 3, and reforming the fuel gas containing water vapor 4 to produce an anode gas using the combustion heat, and a circulation line 7 for supplying a combustion exhaust gas 6 discharged from the reformer 5 to the cathode gas 2a supplied to the fuel cell 3.
Further, the fuel cell power generation apparatus includes a plurality of flow rate regulator valves 8a to 8d, and blowers 9a to 9c. The flow rate regulator valves 8a to 8d and the blowers 9a to 9d are controlled individually by feedback control. Further, based on output instructions at the time of load changes, the sizes of the openings of the flow rate regulator valves 8a to 8d and the rotation numbers of the blowers 9a to 9c in correspondence with the output instructions are calculated by an arithmetic and control device, and prior to the feedback control, the sizes of the openings of the flow rate regulator valves 8a to 8d and the rotation numbers of the blowers 9a to 9c are determined based on the calculation results.
Further, in a solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2003-086225, by directly using hydrocarbon fuel, partial oxidation reaction of the hydrocarbon fuel is used preferentially as power generation reaction. The solid oxide fuel cell includes a water vapor supply apparatus as means for supplying water vapor such that the ratio of steam (S) to carbon (c) (S/C) is regulated to be greater than 0, and 0.5 or less (0<S/C≦0.5). Further, the solid oxide fuel cell includes an adaptive control unit, a carbon deposition prediction/detection unit, and other fuel cells.
However, in Japanese Laid-Open Patent Publication No. 07-022045, though the feedback control is implemented by calculating the sizes of the openings of the flow rate regulator valves 8a to 8d and the rotation numbers of the blowers 9a to 9c based on the output instructions at the time of load changes, the order of these steps is not defined. Therefore, at the time of load reduction, for example, if the flow rate of the cathode gas 2a is reduced first, oxidation of the separator due to the excessive increase in the temperature of the fuel cell power generation apparatus, and degradation of the MEA due to air depletion may occur.
Further, Japanese Laid-Open Patent Publication No. 2003-086225 has an object of only suppressing carbon deposition at the time of load changes, and it is not possible to suitably solve problems other than carbon deposition, i.e., it is not possible to suppress excessive increase in the stack temperature, air depletion or the like. Therefore, for example, at the time of load reduction, if the flow rate of the fuel gas is reduced first, fuel depletion due to the excessive increase in the fuel utilization ratio may occur.